1951 Plymouth deceleration POP

[hep2jive]

Hello everyone! Happy 4th to those in the US. Cheers, elsewhere!

 

So I have a 1951 Plymouth Belvedere. I had the motor rebuilt in '22 and everything seems fine- until its not.

 

When I'm in neutral, I rev the engine it pops as it decelerates. Also driving at higher speeds it pops decelerating.  I have noticed a louder grumble from the exhaust. I replaced the muffler and it is still grumbling. Now it's popping sometimes on cold turn on (thought that could be from me having the gas pedal down and it's popping again from decel) I have a feeling this grumbling and now popping are some kind of vacuum leak or is the float too high? only happens on deceleration.

 

Also this is my only car and is my daily driver. I go out on multiple 500 mile trips and had my valves adjusted  for higher speeds at longer periods of time. i think .13? could this be doing it? Thank you so much for all of your help!

Edited July 3 by hep2jive

[plymouthcranbrook]

You could have a valve out of adjustment probably an exhaust one. You might just have a leak in one of your pipes or the exhaust manifold.  Too much fuel igniting in the pipe from incomplete combustion from rich mixture or a timing issue. Probably other causes as well. Just do a good check on everything and make sure settings are correct first.

[hep2jive]

1 minute ago, plymouthcranbrook said:

You could have a valve out of adjustment probably an exhaust one. You might just have a leak in one of your pipes or the exhaust manifold.  Too much fuel igniting in the pipe from incomplete combustion from rich mixture or a timing issue. Probably other causes as well. Just do a good check on everything and make sure settings are correct first.

For now would then leaning the carb out help? I guess I'll just check everything.

[plymouthcranbrook]

I would think a vacuum leak or a leak in one of the pipes is more likely. How do your plugs look?  And what is the condition of your tailpipe?  Sooty? Or clean.

[plymouthcranbrook]

If it is running well otherwise the mixture is likely ok. The plugs and tailpipe will tell

[hep2jive]

The tailpipe is sooty but it has been for a while. The popping happened right after I had  the valves adjusted and opened up for long distance 55 mph driving. The grumbling has been going for months. Idles smoothly though just had the decel pop and the almost nosy grumbling tail pipe not a smooth hum like it has been. 

[Doug&Deb]

Try going back to the original valve lash specs. The service manual recommends.002 larger on the exhaust valves for high speed driving. I’d also suggest a leak down test and compression test as well as checking all your tune up specs. Beautiful car by the way!

[Sam Buchanan]

Something to consider.....I've found my '48 P15 to be sensitive to idle mixture adjustment and it needs seasonal adjustment to achieve the smoothest idle. Just a small tweak will even out the idle and eliminate most random pops. I do this a couple of times a year when ambient temps change from winter to summer and then back in the fall. I've also noticed this adjustment can be felt at highway speed when the throttle is barely cracked open. Too lean and the engine can have a very slight hesitation, not enough to hear but can definitely be felt. The service manual mentions that idle mixture can impact highway performance under light throttle, that must be what I've observed.

 

Give this a look to make sure it isn't contributing to what you are seeing.

 

 

Edited July 4 by Sam Buchanan

[FarmerJon]

On 7/3/2024 at 3:27 PM, hep2jive said:

 

Also this is my only car and is my daily driver. I go out on multiple 500 mile trips and had my valves adjusted  for higher speeds at longer periods of time. i think .13? could this be doing it?

Beware of thousandths. They are 3 places, so it is best to keep them as 3 places, even if the last zero seems inconsequential. 

.013 (thirteen thousandths) is significantly different from .130 (one hundered thirty thousandths). 

Not trying to be a dick, but it can really screw you up, and I have seen guys get burned by this.

You text your mechanic to set the valves at .13 and you are in for trouble, while .013 is just fine. 

 

As far as the popping: 

Check for leaks in the exhaust, manifolds, vacuum diaphram on distributor, line to vacuum wiper, and in between the sections of the carb body and throttle shaft. 

 

Check your ignition timing. Factory setting is usually 0* but try 4-6*. Make sure timing is advancing correctly, and that your points plate can move freely. Some distributors have a ball bearing for the plate to piviot on, and it can sieze up.

 

My Brockman Mellowtones snap and pop on decel all the time. Fuel mixture can make it better or worse, but ultimately it is the design of the mufflers.

[hep2jive]

Thank you I'll look into it and give an update 

[hep2jive]

It all started with the exhaust note starting to sound more rough but idle was smooth no vibration. Exhaust note used to be pretty smooth. Idle is still smooth just decel pop and rough sounding exhaust. I'll change the manifold gaskets and see if that helps and I'll put the valves back to normal specs.